This application relates to the acquisition of image data by digital cameras and other electronic image acquisition devices, and, more specifically, to detecting the presence of a defined type of object within the image.
Electronic cameras image scenes onto a two-dimensional sensor such as a charge-coupled-device (CCD), a complementary metal-on-silicon (CMOS) device or other type of light sensor. These devices include a large number of photo-detectors (typically two, three, four or more million) arranged across a small two dimensional surface that individually generate a signal proportional to the intensity of light or other optical radiation (including infrared and ultra-violet regions of the spectrum adjacent the visible light wavelengths) striking the element. These elements, forming pixels of an image, are typically scanned in a raster pattern to generate a serial stream of data representative of the intensity of radiation striking one sensor element after another as they are scanned. Color data are most commonly obtained by using photo-detectors that are sensitive to each of distinct color components (such as red, green and blue), alternately distributed across the sensor.
A popular form of such an electronic camera is a small hand-held digital camera that records data of a large number of picture frames either as still photograph “snapshots” or as sequences of frames forming a moving picture. A significant amount of image processing is typically performed on the data of each frame within the camera before storing on a removable non-volatile memory such as a magnetic tape cartridge, a flash memory card, a recordable optical disk or a hard magnetic disk drive. The processed data are typically displayed as a reduced resolution image on a liquid crystal display (LCD) device on the outside of the camera. The processed data are also typically compressed before storage in the non-volatile memory in order to reduce the amount of storage capacity that is taken by the data for each picture frame.
The data acquired by the image sensor are typically processed to compensate for imperfections of the camera and to generally improve the quality of the image obtainable from the data. The correction for any defective pixel photodetector elements of the sensor is one processing function. Another is white balance correction wherein the relative magnitudes of different pixels of the primary colors are set to represent white. This processing also includes de-mosaicing the individual pixel data to superimpose data from spatially separate monochromatic pixel detectors of the sensor to render superimposed multi-colored pixels in the image data. This de-mosaicing then makes it desirable to process the data to enhance and smooth edges of the image. Compensation of the image data for noise and variations of the camera optical system across the image and for variations among the sensor photodetectors is also typically performed within the camera. Other processing typically includes one or more of gamma correction, contrast stretching, chrominance filtering and the like.
Electronic cameras also nearly always include an automatic exposure control capability that sets the exposure time, size of its aperture opening and analog electronic gain of the sensor to result in the luminescence of the image or succession of images being at a certain level based upon calibrations for the sensor being used and user preferences. These exposure parameters are calculated in advance of the picture being taken, and then used to control the camera during acquisition of the image data. For a scene with a particular level of illumination, a decrease in the exposure time is made up by increasing the size of the aperture or the gain of the sensor, or both, in order to obtain the data within a certain luminescence range. An increased aperture results in an image with a reduced depth of field and increased optical blur, and increasing the gain causes the noise within the image to increase. Conversely, when the scene is brightly lighted, the aperture and/or gain are reduced and compensated for by increasing the exposure time, the resulting image having a greater depth of field and/or reduced noise. In addition to analog gain being adjusted, or in place of it, the digital gain of an image is often adjusted after the data have been captured.
Other processing that may also be performed by electronic cameras includes a detection of the likelihood that a certain type of object is present within the image. An example object is a human face. When there is a likelihood that the object is present in the image, its location is also determined. This allows the camera to act differently upon that portion of the image during acquisition and/or processing of the acquired data.